I'm rebuilding an old Heathkit IT-28 Capacitor/Resistor checker. It uses a Wheatstone bridge design with a Magic Eye tube for null indication. It is a fairly simple design with a rectifier tube and a multi-element tube for the only active element (it contains two rectifiers and a triode), and a Magic Eye tube.

This is a Magic Eye tube, a null is indicated when the dark portion is at a minimum:

Measurement range is from 10pF to 1000uF, or compared against an external standard capacitor. Resistance from 5 ohms to 50M ohms, or compared against an external standard. Test signal is 10Vac at 60Hz, or an external sine wave generator can be used, frequency up to 10kHz.

It can also measure inductors but only with an external reference, ratio up to 25:1.

The biggest issue with these is that Heathkit provided some paper capacitors that do not age well, and poor assembly/soldering by whomever put it together. This particular Heathkit appears to be put together well, and it looks like some of the capacitors were upgraded at assembly time.

However, it blew the 1/2A slow blow fuse when powered up. So I suspect the aluminum electrolytic capacitors in the power supply have gone bad. It has two 40uF 350V capacitors in series to get the voltage rating, and giving a total capacitance of 20uF. This is subject to problems when the capacitors are slightly different values due to tolerances. With older leaky caps, it will tend to even out the voltage. If I were to replace them with modern 47uF electrolytics, I'd add 1M to 10M resistors in parallel to ensure equalized voltages.

I had some other things to order from Mouser, so I ordered two from them. It is $7.99 for Priority Mail USPS, or ground UPS or Fed Ex shipping.

The 6AX4 rectifier tube is looking possibly not 100%, based on the internal silvering starting to go black around the edges. I have some 1N4007 1kV 1A diodes I can repair it with, while leaving the 6AX4 in place just for looks. The 6BN8 and the 6E5 Magic Eye tube don't have any obvious problems, I'll know more once I can fire it up.

Before I do this, I'll be disconnecting the transformer from the circuit by pulling the 6AX4, putting a fuse back in, and bringing it up slowly on a variac (variable transformer) while monitoring current and voltage.

File comment: View of the inside, tube side. The black block is a film capacitor, 2.0uF 2% that is part of the power factor measuring circuit. No sense replacing this if it isn't damaged. The white cylinder (and another out of sight) are merely smoothing capacitors on the magic eye and are not critical, as long as they are not leaking.20170110_193128.jpg [ 1.14 MiB | Viewed 530 times ]

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File comment: Note the white powder. I think that is zinc oxide from the zinc plating on a lot of the steel parts. Not from submersion or direct contact with water, but likely from just sitting in a garage or drafty space for a few years. The silver plating on the switch contacts has blackened but is clean where the contacts touch.20170110_193145.jpg [ 1.14 MiB | Viewed 530 times ]

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File comment: This is the Range switch, with the reference capacitors and resistors. I'm not sure, but I think the dark grey block, a 0.02uF 2% capacitor is a film capacitor. I'll measure the value. The capacitor just below, the reddish block, it is a mica capacitor and should be fine.

The resistors all seem to be metal film 1% and should be great.20170110_193157.jpg [ 1.17 MiB | Viewed 530 times ]

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File comment: This is the top of the rectifier tube. It should have a spot that looks like a mirror. As you can see, it has become transparent and is blackening. This would indicate a very slow leak of air inside. I may bypass it with a silicon diode.20170110_193209.jpg [ 1.05 MiB | Viewed 530 times ]

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File comment: The front panel is in pretty good shape, except the 10V leakage voltage got scraped off and a missing knob. I have two replacement knobs, the Fender Telecaster tophat selector switches use the same knobs.20170110_193228.jpg [ 1.09 MiB | Viewed 530 times ]

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File comment: The two big aluminum cylinders at top are the two 40uF 350Vdc aluminum electrolytic capacitors that I'll be replacing with one 20uF 900V capacitor. They are in series to get a higher voltage rating.20170110_193241.jpg [ 1.26 MiB | Viewed 530 times ]

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File comment: This is the switch that sets the voltage for leakage testing, all 10% carbon composition resistors. I'll check the calibration, unless they are >20% off I don't plan on replacing them. The contacts look in good shape.20170110_193257.jpg [ 1.31 MiB | Viewed 530 times ]

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File comment: This is the Bridge, Discharge, Leakage mode switch. That is a mylar film capacitor. Looks good.20170110_193324.jpg [ 1.22 MiB | Viewed 530 times ]

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